Particles of Light?
Mark Lesmeister
Pearland ISD Physics
Select graphics from Serway and Faughn, Holt Physics, Holt Rinehart
and Winston, 2002.
PARTICLES OF LIGHT
Is Light a Particle or a Wave
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Some light
phenomena can be
explained with either
a particle model or a
wave model.
Light does many
things that only
waves do.
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Most are the result
of interference
The Young Double Slit
Experiment
slits
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screen
In 1801, Thomas Young investigated the
nature of light using a double slit experiment.
If light consisted of particles, there would just
be two bright spots behind the slits.
The Young Double Slit
Experiment
Source: Wikipedia Commons
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However, Young found that the light formed
a pattern of bright and dark lines, indicating
that light is a wave.
The Young Double Slit
Experiment
Source: Wikipedia Commons

Studying the pattern that light waves make
allows us to determine the frequency of the
waves.
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The frequency is the number of wave peaks that
move past a point in one second.
Infrared Video:
Bats and Wind Turbines
Videos from Boston University Center for Ecology and
Conservation Biology, http://www.bu.edu/cecb/wind/video/
Blackbody Radiation
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All objects emit
electromagnetic radiation.
The frequencies of this
radiation and the amount
emitted depend on the
temperature of the object.
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“Room temperature” objects
emit infrared frequencies.
Very hot objects (like
glowing coals) emit visible
radiation.
The universe, which has a
temperature of 3 Kelvin, is
awash with microwave
radiation.
© Holt. Rinehart and Winston
Quantization of Energy

In the 19th
century,
experiments
were conducted
to measure the
blackbody
radiation emitted
at each
wavelength of
light.
Source: Wikipedia Commons
Quantization of Energy

The wave-theory
of EM radiation
did not agree
with the
experimental
results for
blackbody
radiation.
Source: Wikipedia Commons
Quantization of Energy
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In 1900, Max Planck
developed a new theory
that assumed energy
isn’t emitted in any
amount, but only in
packets, called
“quanta”.
This “quantum” theory
agreed with the
blackbody experiments.
© Holt. Rinehart and Winston
Planck’s Equation
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Planck’s equation relates the energy of
a quantum of light with the frequency
of light.
Energy = Planck’s constant x
frequency
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E  hf
In atoms, energy is measured in
electron volts (eV)
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1 eV = 1.60 x 10-19 J
h=4.14 x 10-15 eV-s
The Photoelectric Effect

When light strikes a
metal surface, it will
give off electrons.
Graphic from Serway and Faughn, “Holt
Physics” © Holt, Rinehart and Winston
The Photoelectric Effect
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If light is a wave, then a
light beam that has
greater intensity
(greater energy) should
cause the electrons
given off to have more
energy.
A more intense light
beam causes more
electrons to be emitted,
but they all have the
same energy.
Graphic from Serway and Faughn, “Holt
Physics” © Holt, Rinehart and Winston
The Photoelectric Effect
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Einstein explained the
photoelectric effect by
assuming that light was
made of particles.
A more intense light
beam has more
photons, but each
carries the same
energy.
Thus, more intense light
would produce more
electrons, with the
same energy.
Graphic from Serway and Faughn, “Holt
Physics” © Holt, Rinehart and Winston
Quantum Theory of Light and
the Photoelectric Effect
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When a photon strikes a metal plate, it
transfers energy to the electrons in the plate.
If an electron acquires enough energy, it can
be ejected from the plate.
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The energy necessary to eject an electron is called
the “work function” of the metal.
The kinetic energy of the electrons will be the
energy of the photon minus the work function.
KE  hf  Work Funct ion
Photoelectric Effect: Practice
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A certain beam of light has photons
with 5 eV of energy. When this light
strikes a metal plate, electrons with
3 eV of kinetic energy are released.
What is the work function of the metal?
Answer: 2 eV
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In some experiments,
such as blackbody
radiation and the
photoelectric effect,
light acts like a particle
(called a photon).
In others, like double
slit interference and thin
film interference, light
acts like a wave.
Light never appears as
both in the same
experiment.
Source: Wikipedia Commons

Graphic from Serway and Faughn, “Holt
Physics” © Holt, Rinehart and Winston
Wave Particle Duality